Assessing the Novel Mixed Tutton Salts K2Mn0.03Ni0.97(SO4)2(H2O)6 and K2Mn0.18Cu0.82(SO4)2(H2O)6 for Thermochemical Heat Storage Applications: An Experimental–Theoretical Study

In this paper, novel mixed Tutton salts with the chemical formulas K2Mn0.03Ni0.97(SO4)2(H2O)6 and K2Mn0.18Cu0.82(SO4)2(H2O)6 were synthesized and studied as compounds for thermochemical heat storage potential. The crystallographic structures of single crystals were determined by X-ray diffraction. Additionally, a comprehensive computational study, based on density functional theory (DFT) calculations and Hirshfeld surface analysis, was performed to calculate structural, electronic, and thermodynamic properties of the coordination complexes [MII(H2O)6]2+ (MII = Mn, Ni, and Cu), as well as to investigate intermolecular interactions and voids in the framework. The axial compressions relative to octahedral coordination geometry observed in the crystal structures were correlated and elucidated using DFT investigations regarding Jahn–Teller effects arising from complexes with different spin multiplicities. The spatial distributions of the frontier molecular orbital and spin densities, as well as energy gaps, provided further insights into the stability of these complexes. Thermogravimetry, differential thermal analysis, and differential scanning calorimetry techniques were also applied to identify the thermal stability and physicochemical properties of the mixed crystals. Values of dehydration enthalpy and storage energy density per volume were also estimated. The two mixed sulfate hydrates reported here have low dehydration temperatures and high energy densities. Both have promising thermal properties for residential heat storage systems, superior to the Tutton salts previously reported.